Capacitor and dielectric material therefor



Jan, 26, 1960 J. R. HUTzLER CAPACITOR AND DIELECTRIC MATERIAL THEREFOR Filed Feb. s, 195e lUnited Staes Parent o relates to capacitors (static elecparticularly, the invention relates Thepresent invention.v 'tn'c'ondensersy More typefor'capacitors. n: l .i MLDielectric materialsor iir'ipreg'iiantsV of ,srolidrresi'iious or plastic type'have been dielectrics for use in-capacitors 'for several reasons.

fcopolymeriaation.` methyl styrene and divinyl benzene. The Walker material, referred to hereinafter as styrene copolymer as disto shrinkage during the curing process. A l This 'styrene copolymer, while superior to other types of known dielectric material,

subjected. y v Y Y i It is an object of the present invention to provide ca- 'pacitors having improved electrical, vthermal and life characteristics. i v v Another object of the present invention is the provision of a solid dielectric material 'of the above type which 'LOther objects and advantages willpbecomebapparent v lovw'ng description panying drawing, in

tjo' an improved dielectric material of a solidified resinous 3 mixture of styrene, polyalp'ha fromme-following description'and the appended claims. 1

2,922,937 ce Patented Jan. 26, 196? l The above objects and others are attained in accorde ance with the invention by is preferably present to about 1-10% other embodiments of the invention, the ture also contains styrene and/or alpha in .monomeric or polymeric form. Y Tlie invention will be better understood from the fol- -taken in conjunction with the accomwhich: u

Fig. l is a perspective View of a partially unrolled capacitor assembly of a type iii which the present invention may be suitably embodied; and

4VFig. 2 is a view factory. At higher voltages additional paper sheets may be used. While kraft paper spacers are preferred, other types of spacer material may be used, such as polyeththe invention involves in its outer surface ofthe container may be cleaned of all resinous material Awhile it is still in the liquid state. The curing cycle conditions depend upon the particular resin composition used and the size of the capacitor. A suitable cure cycle used in accordance with the invention may consist of slowly raising the temperature from 25 C. to 125 C. over a Iperiod of l4 hours followed by 4 hours at 125 C. f Y In accordance with an embodiment ofethe invention it has been found that capacitors .of superior properties may be made by impregnating Vthe paper spacers 3, 4 witha copolymerizable mixture comprising at least-vinyl toluene and about l-10% by weight `of divinyl benzene. While broad aspects a `copolymer ofthe above materials, the starting mixture preferably also containsrsuitable proportions of styrene and alpha methyl styrene, the latter material being either in lpolymeric or monomeric form. 'In general, the proportions ofthe constituents which may be used in accordance with the invention are, in percent by Weight, l-98% ofzvinyl toluene, 90% styrene, 0-50% alpha methylj styrene `(monomer or polymer), and 1-10% divinyl benzene. A small amount of catalyst such as tertiary butylY hydroperoxide or other suitable known accelerators or catalysts may also be present.

The unsatisfactory Vresistance of the styrene copolymer .dielectric-materialreferred to above to thermal shock has been attributed to the Vvbrittleness of the dielectric vresin impregnant and its shrinkage during polymerization. Preventive measurestaken inthe past to avoid'these conditions included ,lowering of the concentration of divinyl benzene in the starting mixture so that the resulting copolymer does not become brittle. Also, relatively large amounts '(fe.g. 40%) of alpha methyl polystyrene have been added as an extender to reduce shrinkage and also as a plasticizer. In view of thisY prior art teaching, it was surprising to find that excellentV thermal shock characteristics could beobtained with the present dielectric material using high concentrations `ot vdivinyl benzene and in the absence of alpha methyl polystyrene. Styrene itself has tions of the present invention while still obtaining satisv.factory results. 'Furthen not only are excellent thermal shock properties obtained, but also electrical characteristics such as life, insulation resistance, capacitance 4,and power factor have beenv produced which are superior to those of styrene copolymer and other known solid dielectrics. f The vinyl toluene used in the invention is a mixture of isomers and is added in the form of a liquid. A typi'- lcal composition of this material contains, in percent by weight, V60% meta-vinyl toluene and 40% para-vinyl toluene. d .Y In general, the proportion of divinyl benzene used in preparing the copolymer is about l-% the mixture. Concentrations of divinyl benzene Y'below that range will not result in sufficient cross-linking to produce a proper copolymer, While higher concentrations appear to cause undesirable embrittlement of the final product. In a typical commercially available composition there is present a mixture of isomers of divinyl benzene, diethyl benzene and ethyl Vinyl benzene, the divinyl benzene being present 1n such a mixture in an amount of about 50-60% by weight. The materials 'in Vthe commercial mixture other than divinyl benzene, however, are inert in so far ascross-linking or thermosetting properties are concerned in the present invention.

A typical commercial sample of divinyl benzene has the following compositionin percent by weight:

f Y 'Pefcent Divinyl benzene v55;() Ethyl vinyl benzene '35.4' Bethyllbenzene V "9:6

by weight ofv been omitted from composiy EXAMPLE I Y I Percent Styrene 65 Divinyl-benzene solution) l0 Alpha methyl styrene 15 Vinyl toluene K 10 EXAMPLE n Y Y r Percent Styrene v 75 Divinyl benzene (60%.solution) 10 Vinyl toluene 15 EXAMPLE III Y r s Percent Styrerre V f 65 Divinyl benzene (60% solution) 5 Alpha methyl styrene S Vinyl toluene r 5 EXAMPLE IV `Percent Styrene f l,.65 Divinyl benzene `(60%-solution) y 41-5 Vinyl toluene Y f 2.0

A EXAMPLE V Percent Vinyl toluene v6755 `Divinylbenzene (60% solution) 7-.5 Alpha methyl polystyrene 25:0 EXAMPLE v1 Y Percent Y 'Styrene Y 55, Divinyl benzene (60% solution) 1,0 Alpha methyl polystyrene 15 Vinyl toluene y 10 EXAMPLE `VII Y Y vrBercent Vinyl toluene. Y Y l 9() Divinyl benzene Y 10 thereby to limit the scope ojf ther invention in any way:

lIn preparing the .various mixtures ,ofthe present dielectric material for testing in accordance with the pm,- vcedures described below, each of the tested mixtureswas catalyzedwith 0.3% vtertiary butyl hydroperoxide. In makingthe `capacitor 4units for the following tests, the capacitors were dried lat 125 C. undervacuum of less than 500 micronsand impregnated with the respectivo "dielectric compositions. The Aimpregnants were Ythen vcured in situ by heat at temperatures rising slowly from 25 C7.te125'c. Y Y -Inthe following TableI, various-.characteristicsof the above dielectric compositions are compared at temperatures `of'25 C., 100 C. v.'.tndl125\ the capacitance and percent power factor being vmeasured at -v.6 ,0

cycles *per second and insulation resistance beingmeasured Yat '500 v."D.C,.` at ll minute electriiication. The values f'Cable'I' Vinyl Toluene copolymers styrene @Pregnant v Y copoly- YV I mer Y I II III IV "wop Cap. mf 0. 97 0. 92 0. 98 1. 0 Percent RF-.. 0.30 `0. 30 0.30 0.40 ningen-mrd.- 16, 500 16, 000 16, 600 12, 000

Cap. mrd.. 0.97 0.99 0. 95 0.99 1.0 Percent P F. 0.49 0. 67 0. 67 0. 54 0. 43 IR meg.mfd.-. 123 82 86 94 51 In a separate test comparing vinyl toluene copolymer composition V with the styrene copolymer, where the characteristics above were measured at 125 C. the cathe same in both copolymers, but the average IR of the composition V was about 24% higher than that of the styrene copolymer. Further, dielectric strength tests made between these two copolymers showed a dielectric rene copolymer.

-It will be evident from the above data that the present capacitance and power factor.

Comparative thermal cycle tests were made on various types of the present copolymers and on the styrene copolymer, this test consisting of the following procedure. The capacitors were maintained at -55 C. for at least one-half hour followed by room temperature conditionthan one-half hour, after which the capacitors were placed in an oven at 125 C. for at least one-half hour. After each of these temperature cycles, the capacitors were given a one-second dielectric test of 1200 v. D.C. at room temperature. It was found that capacitors incorporating the present copolymer materials was only 74 cycles.

In tests made on D.C. position II material was 4,440 v. per mil as compared to 2,666 v. per mil of the styrene copolymer. Life tests made on the same materials wherein the units were subjected to 560 v. D.C. at 125 C. showed that of the units impregnated with the present material only 8% had failed after 2,431 hours, whereas of the units with styrene copolymer dielectric material 50% had failed by that time.

In further tests on low temperature characteristics to determine change in capacitance of the units at 55 C. as compared to 25 C., it was found that the average decrease in capacitors having various compositions of the present vinyl toluene copolymers was 4.2% while that of the styrene copolymer material was 5.4%.

The electrical characteristics of the present dielectrics even when the impregnants were stored for a prolonged period at low temperature were still satisfactory. In one such storage test the following results (as measured at C.) were obtained, the values being an average of 75 6 4several capacitor'unts with the composition ofxaxnpl'e II as impregnant:

Table II .After 16 Initial weeks at -20 C.

Cap-mrd 1.01 99 Percent P.F. 0. 40 0. 30 IR meg.mfd 113 119 prolonged periods of time.

While particular compositions of the present vinyl toluene copolymers have been set forth above, it is not while still producing satisfactory results in accordance with the invention. Moreover, capacitors of other types of construction than that shown, e.g., stacked capacitors, may suitably incorporate the present copolymer dielectric materials, and electrical those skilled in the art without actually departing from the scope of the invention. Therefore, the appended claims are intended to cover all such equivalent variations as come within the true spirit and scope of the invention.

What I claim as new and desire to secure Patent of the United States is:

1. A dielectric sheet material comprising a porous dielectric sheet impregnated with a copolymerized mixture in percent by weight of 590% vinyl toluene, l-10% divinyl benzene, and at least one compound selected from the group consisting of styrene, alpha methyl styrene and alpha methyl polystyrene.

2. Electrical apparatus comprising an electrical conductor insulated with a dielectric compound comprising the copolymerizaton product of a mixture in percent by weight of 5-90% vinyl toluene, l-10% divinyl benzene, and at least one compound selected from the group consisting of styrene, alpha methyl styrene and alpha methyl polystyrene.

3. Electrical apparatus comprising, in combination, spaced electrical conductors and insulating material arranged therebetween, said insulating material comprising the solid copolymerization product of a mixture in percent by weight of 5-90% vinyl toluene, 1-l0% divinyl benzene, and at least one compound selected from the group consisting of styrene, alpha methyl styrene and alpha methyl polystyrene.

4. A capacitor comprising, in combination, cooperating armatures and dielectric spacer material therebetween,

by Letters ing armatures and dielectric spacer pound selected fromthe group consistingf-ofalph'a methyl styrene .and alpha methylppolystyrene.

References CtedinY thezfle of this patent UNITED :STATES PA-TENTS Y OTHERREFERENQES t.

'10 A'Boundy-Boyer: and Derivatives, 44.

I Styrene, Reinhold (19521),

pagesv rr1232., 1241- 

4. A CAPACITOR COMPRISING, IN COMBINTION, COOPERATING AMRATURES AND DIELECTRIC SPACER MATERIAL THEREBETWEEN, SAID DIELECTRIC SPACER MATERIAL INCLUDING THE SOLID COPOLYMERIZATION PRODUCT OF A MIXTURE IN PERCENT BY WEIGHT OF 5-90% VINYL TOLUENE, 1-10% DIVINYL BENZENE, AND AT LEAST ONE COMPOUND SELECTED FROM THE GROUP CONSISTING OF STYRENE, ALPHA METHYL STYRENE AND APLHA METHYL POLYSTYRENE. 